1. Technical Field
The present invention relates to a vertical-cavity surface-emitting laser diode (hereinafter referred to as VCSEL) that may be used as a light source of optical data processing or high-speed optical communication and a manufacturing method thereof, and more specifically to a technology of suppressing high-order transverse mode lasing.
2. Related Art
In technical fields such as optical communication or optical storage, there has been a growing interest in VCSEL. VCSELs have excellent characteristics which edge-emitting semiconductor lasers do not have. For example, VCSELs are characterized by lower threshold current and smaller power consumption. With a VCSEL, a round light spot can be easily obtained. Also, evaluation can be performed while VCSELs are on a wafer, and light sources can be arranged in two-dimensional arrays. With these characteristics, demands especially as light sources in the communication field have been expected to grow.
When a VCSEL is coupled to an optical fiber, it is desirable that laser light be in a single transverse mode or fundamental transverse mode. This is because single transverse mode has a smaller radiation angle and higher efficiency in coupling with an optical fiber or the like, than multi-mode has. Therefore, there have been proposals to suppress high-order transverse mode of laser light emitted from a VCSEL.
For example, as shown in FIG. 18, a VCSEL includes a reflectivity adjusting layer 12 in the top layer of an upper mirror 10. The reflectivity adjusting layer 12 has a first mirror portion 12a having a thickness (thickness of λ/4) that meets the Bragg reflection condition, and a second mirror portion 12b having a thickness (thickness of λ/2) that meets the anti-Bragg reflection condition. Thus the reflectivity of the first mirror portion 12a is made greater than the reflectivity of the second mirror portion 12b. The first mirror portion 12a is formed in a center portion of the optical axis of an opening portion 14a of an upper electrode 14, and the second mirror portion 12b in a peripheral portion being away from the optical axis suppresses high-order transverse mode lasing, and thus single transverse mode lasing is obtained.
The structure of VCSEL of related art described above have a certain effect of suppressing high-order transverse mode, however, they do not always sufficiently suppress high-order transverse mode in a wide temperature range. When VCSELs are operated at a low temperature, there is a problem in that lasing starts from high-order transverse mode. The lasing increases lasing threshold value of low-order transverse mode, and thus lasing in the low-order transverse mode is less prone to occur. On the other hand, when VCSELs are operated in a high temperature, optical output is significantly reduced as compared with the case of at room temperature. Such problems are not known nor solved in any of related arts.
In addition, the suppression of high-order transverse mode proposed in the related art requires an etching process that may cause variations in thickness or cause steps in the reflectivity adjusting layer, and thus it is difficult to form an accurate thickness with a high degree of reproducibility, by the etching.
According to other related art, they suppress high-order transverse mode by the shape of the upper electrode. Therefore the shape of the upper electrode should be processed into a shape that matches to the shape of the dark portion of the emission pattern, and the processing is quite complicated. Furthermore, the shape corresponds to a specific transverse mode, and thus it is difficult to sufficiently perform suppression of high-order transverse mode.
An object of the present invention is to address the issues of related arts described above, and provide a VCSEL that is capable of suppressing high-order transverse mode in a wide temperature range. Another object of the present invention is to provide a method of manufacturing a VCSEL that is capable of suppressing high-order transverse mode without adding a complicated process.